Template factorial function without template specialization - c++

Question

I don't understand the following behavior.
The following code, aimed at computing the factorial at compile time, doesn't even compile:
#include <iostream>
using namespace std;
template<int N>
int f() {
if (N == 1) return 1; // we exit the recursion at 1 instead of 0
return N*f<N-1>();
}
int main() {
cout << f<5>() << endl;
return 0;
}
and throws the following error:
...$ g++ factorial.cpp && ./a.out
factorial.cpp: In instantiation of ‘int f() [with int N = -894]’:
factorial.cpp:7:18: recursively required from ‘int f() [with int N = 4]’
factorial.cpp:7:18: required from ‘int f() [with int N = 5]’
factorial.cpp:15:16: required from here
factorial.cpp:7:18: fatal error: template instantiation depth exceeds maximum of 900 (use ‘-ftemplate-depth=’ to increase the maximum)
7 | return N*f<N-1>();
| ~~~~~~^~
compilation terminated.
whereas, upon adding the specialization for N == 0 (which the template above doesn't even reach),
template<>
int f<0>() {
cout << "Hello, I'm the specialization.\n";
return 1;
}
the code compiles and give the correct output of, even if the specialization is never used,
...$ g++ factorial.cpp && ./a.out
120

Answer 1

The issue here is that your if statement is a run time construct. When you have
int f() {
if (N == 1) return 1; // we exit the recursion at 1 instead of 0
return N*f<N-1>();
}
the f<N-1> is instantiated as it may be called. Even though the if condition will stop it from calling f<0>, the compiler still has to instantiate it since it is part of the function. That means it instantiates f<4>, which instantiates f<3>, which instantiates f<2>, and on and on it will go forever.
The Pre C++17 way to stop this is to use a specialization for 0 which breaks that chain. Starting in C++17 with constexpr if, this is no longer needed. Using
int f() {
if constexpr (N == 1) return 1; // we exit the recursion at 1 instead of 0
else return N*f<N-1>();
}
guarantees that return N*f<N-1>(); won't even exist in the 1 case, so you don't keep going down the instantiation rabbit hole.

Answer 2

The problem is that f<N>() always instantiates f<N-1>() whether that if branch is taken or not. Unless properly terminated, that would create infinite recursion at compile time (i.e. it would attempt instantiating F<0>, then f<-1>, then f<-2> and so on). Obviously you should terminate that recursion somehow.
Apart from constexpr solution and specialization suggested by NathanOliver, you may terminate the recursion explicitly:
template <int N>
inline int f()
{
if (N <= 1)
return 1;
return N * f<(N <= 1) ? N : N - 1>();
}
Mind, this solution is rather poor (the same terminal condition must be repeated twice), I'm writing this answer merely to show that there are always more ways to solve the problem :- )